Rotating vehicle accessories, such as the alternator, are powered by an engine driven belt that is subject to the same speed variations and the same accelerations and decelerations as the engine. Generally, the alternator is directly driven by the belt through a pulley fixed to the alternator shaft. The belt wraps and grips the pulley, rotating it and the alternator shaft in one direction. This creates a tight and a slack belt side. Two problems are created by direct drive. One is the problem of efficiently running the alternator. At low engine and belt speeds, it is desirable that the pulley ratio be as large as possible. This assures that the alternator output, which is proportional to its speed, will be sufficient at low belt speeds. The alternator cannot be run too fast without damage, however, so there is an upper limit on the pulley ratio.
Another problem arises from the fact that an alternator has a significant resistance to being rotated, especially at high rates of acceleration, due both to its rotational inertia and the electrical resistance that any generator has to being turned. Likewise, the rotating alternator resists any force attempting to quickly brake it. The belt turns the pulley only by virtue of the strength of its grip thereon. If the alternator's resistance to high acceleration exceeds the belt grip, the belt can slip on the pulley, causing a fleeting but unpleasant noise called belt "chirp". Likewise, alternator resistance to being rapidly braked can cause belt slip in the other direction. In fact, since belt acceleration forces are directed toward the tight side of the belt and deceleration forces toward the slack side, the belt is less able to brake the alternator without slipping than it is to accelerate it, even though acceleration forces are generally greater in magnitude.
Various alternator drives have been proposed that would provide one-to-one drive at low speeds, but less than one-to-one at high pulley speeds, so as to allow the use of a large pulley ratio without exceeding the upper speed limit. These include electromechanical clutches, continuously variable transmissions, viscous clutches and continuously variable transmissions. Likewise, various torque limiting devices have been proposed to prevent belt chirp, such as overrunning clutches. None has found real commercial acceptance, because of cost, size and complexity. A practical and compact indirect drive clutch that solved both problems would be useful for alternators or other accessories.